[foreign] RFR: Remove warnings when building tests

[Jorn Vernee]

Hi,

Previously jdk/foreign test were being built with AVX2 flags added, but 
these flags were removed to support more platforms.

However, the test dir is still being redundantly added to the jtreg 
native source dir list, which generates a lot of warnings about 
'ignoring old recipe' when running tests.

This is a small fix which removes the redundancy.

Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/testwarnings/webrev.00/

As you can see in the webrev, the enclosing `$(TOPDIR)/test/jdk` is 
already added as source dir, and it is searched recursively, so adding 
the java/foreign sub dir separately is not needed.

Thanks,
Jorn